In the removal of sulfur dioxide and other toxic and noxious components from flue gases before they are subjected to reuse, to further treatment, or to discharge into the atmosphere, it is known to employ dry absorbents which interact with these components and form reaction products which are thereby eliminated upon the removal of particulates from the gas stream.
These particulates, after removal from the remainder of the flue gas, constitute a residue which must be disposed of.
Dry absorbents for the desulfurization of flue gases, especially the flue gases of boiler combustion chambers and power plant boilers in particular, can comprise calcium-containing compounds such as calcium oxide (CaO), calcium carbonate (CaCO.sub.3) and the calcium hydroxide (Ca[OH].sub.2). These absorbents can be used alone or together with corresponding magnesium compounds.
The absorbent can either be introduced into the fuel so as to be entrained into the combustion chamber therewith and begin the reaction phase with the sulfur dioxide as it is formed or introduced into the flue gases in the combustion chamber or downstream thereof.
In the commonly assigned U.S. Pat. No. 4,262,610 (corresponding to German patent DO-PS No. 28 07 076) the absorbent is combined with the low-rank solid fuel by dry milling the two together.
The primary reaction product from the use of such absorbents is the calcium sulfate (CaSO.sub.4) also known as anhydrite.
The product, together with other particulates, can be recovered in an electrostatic filter or by some other particle removal technique and the residue contains, apart from residual absorbent and its reaction products with the toxic or noxious components of the flue gas (e.g. anhydrite), substantial quantities of fly ash. Depending upon the sulfur content of the flue gas, the fly ash proportion can be between 15 and 60% by weight and generally is about 40% of the residue. The fly ash content can vary significantly especially when the fuel is a low-rank coal such as brown coal and complicates the process since it is desirable to keep the anhydrite concentration in the residue constant over long periods of time to permit effective use of the residue.
The residue can be stored at a dump site, especially if the anhydrite concentration varies, to allow the accumulation of large quantities, and blended where a varying composition is involved. Not only does this require considerable space but it also creates an environmental hazard since leaching from the residue is possible under ambient conditions.
When the anhydrite concentration is insufficient or varies, the product does not have the constancy required for use as a building material or the like.